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Tag Archives: Huawei router

ISIS DIS election

Let’s check today how a Designated Intermediate System (DIS) is elected in broadcast network.

Unlike in OSPF, in ISIS, routers of the same level, in a network segment, set up adjacencies (including non-DIS routers). DIS, as a pseudo node also generates LSPs, to describe available routers in the network. A pseudo node is indentified by the system ID of the DIS and the 1-byte circuit ID, which is always not 0. The main task of DIS is to reduce the number of generated LSPs. Even though all routers set up adjacencies in an ISIS broadcast network, LSDBs are synchronized by the DISs. One router can be DIS either for Level -1 or Level 2 routers. It depends on DIS priority configured. You can configure different DIS priority for different levels. The highest priority means the router is elected as DIS. In case that all routers have the same DIS priority, the one with the highest MAC address is chosen as DIS. By default DIS priority is set to 64 and can be changed manually. Comparing to OSFP DR, the router with the priority 0 also takes part in DIS election. Each time you connect a new router with higher DIS priority, the new router is elected as the new DIS, which causes LSPs flooding.

Let’s leave theory and carry out a test of DIS election in ISIS broadcast network.

Based on the below topology configure, IP addresses for physical and loopback interfaces (it is omitted here).

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ISIS route aggregation

Let’s keep going and try to configure ISIS route aggregation based on the following topology:

If you want to recall how to configure ISIS adjacency on Huawei routers, just go to ‘ISIS on Huawei routers‘.

To avoid DIS election, configure all physical interfaces as ISIS point-to-point (p2p) links (Router 1 as an example):

[1-Ethernet0/0/8]isis circuit-type p2p

[1]dis isis interface 

                       Interface information for ISIS(1)
 Interface       Id      IPV4.State          IPV6.State      MTU  Type  DIS   
 Eth0/0/8        003         Up                 Down         1497 L1/L2 -- 
 Loop0           001         Up                 Down         1500 L1/L2 -- 
 Loop100         002         Up                 Down         1500 L1/L2 -- 

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ISIS on Huawei routers

ISIS is a link-state IGP protocol. It gathers routing information from adjacent neighbors and uses the SPF algorithm to determine the best paths to destinations.

I wouldn’t like to focus on the theory because you can find it in many sources.

Let’s configure ISIS protocol based on the following topology:

Configure IP addresses of physical and loopback interfaces on all routers (it is omitted here):

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from Huawei CLI – lock and send

Today a few words about 2 simple but useful commands: lock and send.

LOCK – prevents unauthorized users from operating on the current terminal interface

SEND – enables the system to transfer messages between user interfaces

Let’s look how they work on Huawei S5700 switch.

Enter Password:
Confirm Password:

 Info: The terminal is locked. 

Enter Password:


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OSPF virtual link

OSPF virtual link is a tunnel that extends backbone area through a non-backbone area. It is interpreted by the router as unnumbered point-to-point network. Virtual links must be configured between two Area Border Routers. These ABRs are considered neighbours, by establishing the virtual link between them, although they are not linked physically. The transit area (the area through which the virtual link is configured) must have full routing information and cannot be a stub area.

Virtual links add complexity to a network and should be avoided. Use them only as a temporary fix to OSPF topology problem.

Virtual links are used for the following purposes:

  • To link an area to the backbone area through a non-backbone area (most common case). On the picture below Area2 was connected to the Area0 via virtual link, which extends Area0 through transit Area1.

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